Robert Downey Jr. Says Cillian Murphy Made Biggest Sacrifice of Any Actor He’s Ever Seen for ‘Oppenheimer’ (Exclusive)

Like many who have seen Oppenheimer, Robert Downey Jr. was floored with Cillian Murphy’s transformative performance as J. Robert Oppenheimer, the American physicist who led the U.S. effort to create an atomic bomb during World War II.

Speaking with PEOPLE for this week’s issue, Downey, 58, said of his Oppenheimer costar, “I have never witnessed a greater sacrifice by a lead actor in my career.”

Downey, who plays Atomic Energy Commission chairman Lewis Strauss in the film, says he was impressed by Murphy’s “commitment” to the massive undertaking of leading Christopher Nolan’s three-hour epic.

“He knew it was going to be a behemoth ask when Chris called him. But I think he also had the humility that is required to survive playing a role like this,” says Downey.

“We’d be like, ‘Hey, we got a three-day weekend. Maybe we’ll go antiquing in Santa Fe. What are you going to do?’ ‘Oh, I have to learn 30,000 words of Dutch. Have a nice time.’ But that’s the nature of the ask.”

Florence Pugh, 27, who shares most of her scenes with Murphy as Jean Tatlock, one of the major loves of Oppenheimer’s life, offered further praise.

“Chris had one of the most incredible leads in Cillian,” she tells PEOPLE. “He is an actor that I have been watching for quite some time and have been desperate to work with for ages. You’d have to be mad to say no. It was truly one of the best experiences that I’ve had.”

“Working with him was hugely impressive,” she continues. “Every single day he shows up knowing every single possible way, intonation, inflection of how to bring this character to life. That was hugely impressive to me. There’s a reason why he is one of the greats.”

Murphy’s commitment to the role included following a very restrictive diet to closer resemble Oppenheimer.

His costar Emily Blunt, who plays his onscreen wife Kitty Oppenheimer, recently told Extra, “He had such a monumental undertaking. And he could only eat, like, an almond every day. He was so emaciated.”

The Irish actor addressed his dramatic weight loss for the role during a recent interview with IndieWire, stressing, “I don’t advise it.”

Oppenheimer is now playing in theaters.